This invention relates generally to computers and more particularly to circuitry for computers for producing color video displays.
Various microcomputers are commercially available to home or business applications and include circuitry for producing a color video display on either a color monitor or a standard color television receiver. For example, color video generation circuitry is included in the Apple II.RTM. computers sold by Apple Computer Company, Inc. of Cupertino, Calif. The color video producing circuitry used in such computers is shown and claimed in U.S. Pat. No. 4,278,972. That circuitry basically utilizes a shift register combination to sequentially sample individual bits making up an eight bit data code representing a color provided by the computer's random access memory (RAM). The data code representing the color is introduced broadside into two, four-bit shift registers which are each clocked at a frequency of 14.31818 MHz. Each register sequentially shifts the input nibble (four bits) through its stages, with the output of the last stage being coupled back to the input of the first stage. Accordingly, the output of the shift register consists of plural bits representing the data code for the color and appearing at a frequency of 3.579545 MHz (the NTSC standard color subcarrier reference frequency). The digital output signal of the shift register is sequentially sampled, with each bit being used to either turn the video on or off in synchronism with the color reference signal of the receiver. Thus, for each high or "one" bit, the video generator produces an output signal to the receiver's phase demodulator. Conversely, for each low or "zero" bit, no output signal is produced.
The sequential sampling of the data code shifted through the recirculating shift registers has the effect of inherently producing different colors for different data codes. In this regard, as the bits are recirculated a nibble at a time, each cycle of the subcarrier color reference signal is divided into four sectors or quadrants, with successive bits of the nibble representing successive quadrants of the color cycle. Thus, for each bit of the nibble which is high a signal representative of the associated quadrant portion of one cycle of the color subcarrier reference signal is passed to the phase demodulator of the receiver. The demodulator operates in synchronism with the video generator so that the signal produced by the phase demodulator constitutes the portion of the primary color signals (red, blue and green) making up the subcarrier signal in that quadrant, i.e., the appropriate 90.degree. portion of one cycle of the color subcarrier reference signal. If any of the bits in this nibble is low durinng that quadrant of the color subcarrier reference signal, no video is produced.
As will be appreciated, the portions of the primary color signals in the first quadrant of the standard color subcarrier reference signal are 60.degree. of the red signal component and 30.degree. of the blue signal component. The second quadrant (or the next 90.degree. portion of the color subcarrier signal) represents 90.degree. of only the blue signal component. The next 90.degree. of the color subcarrier (that is the third quadrant) represents 90.degree. of only the green signal component, while the last 90.degree. of the color subcarrier signal (that is the fourth quadrant) represents 30.degree. of the green signal component and the remaining 60.degree. of the red signal component.
The output of the receiver's phase demodulator thus constitutes a signal representative of those portions of the primary color signals present during the four quadrants making up one color cycle and thereby produces a unique color.
Owing to the shift register arrangement as described above, each data code produces an inherently predetermined color and that color alone. However, some color codes (nibbles) do not produce unique colors. For example, the binary color code of 1010 (representing the decimal number 10) and the binary color code 0101 (representing the decimal number 5) both produce the same shade of gray in the Apple II.RTM. computer.
As will be appreciated from the foregoing, while the video generator disclosed in U.S. Pat. No. 4,278,972 provides a simple means for the production of up to 16 colors from corresponding digital data codes, such a generator leaves much to be desired from the standpoint of functionality, effectiveness and compatibility with other computers.